Polyphase permanent magnet, brushless motors driven by a sinusoidal current offer the capability of providing low torque ripple, noise and vibration in comparison with those driven by a square wave current. Theoretically, if a motor controller can produce polyphase sinusoidal currents with the same frequency as that of the sinusoidal back EMFs, the torque output of the motor will be a constant, and zero torque ripple can be achieved. However, due to practical limitations of motor design and controller implementation, there are deviations from pure sinusoidal back EMF and current waveforms. The deviations will typically result in parasitic torque ripple components at various frequencies and magnitudes.
Another component of torque ripple in a conventional permanent magnet, brushless motor is cogging torque. Cogging torque is a result of the magnetic interaction between the permanent magnets of the rotor and the slotted structure of the armature. As the leading edge of a magnet approaches an individual stator tooth, a positive torque is produced by the magnetic attraction force exerted therebetween. However, as the magnet leading edge passes and the trailing edge approaches, a negative torque is produced. The instantaneous value of the cogging torque varies with rotor position and alternates at a frequency that is proportional to the motor speed and the number of slots. The amplitude of the cogging torque is affected by certain design parameters such as slot opening/slot pitch ratio, magnet strength and air gap length.
One approach to reducing torque ripple is to employ a slotless armature, which allows for precise winding patterns in order to achieve a pure sinusoidal back EMF. In addition, the absence of slots in the armature eliminates the cogging torque resulting therefrom. However, the manufacturing process for slotless motors is not well defined and thus the manufacturing costs thereof may be prohibitive.
The problems and disadvantages of the prior art are overcome and alleviated by a permanent magnet structure for use in brushless motors. In an exemplary embodiment of the invention, the magnet structure includes a parallelogram shaped body. The body has an outer surface and an inner surface, with the outer surface and the inner surface being arcuate in shape.
In a preferred embodiment, the outer surface and the inner surface are generally concentric with one another. The body is preferably comprised of neodymium-iron-boron material and is epoxy coated. In an alternative embodiment, the body is nickel-plated. In still another alternative embodiment, the body is aluminum deposition coated.